1. Field of the Invention
The present invention relates to an exhaust gas sampling apparatus for analyzing contents in exhaust gas from an automobile, a motorboat, a mowing machine and so on.
2. Description of Related Art
The construction of an exhaust gas sampling apparatus according to the conventional art will be explained by referring to FIG. 8. In the figure, reference numeral 201 indicates an intake air supply conduit for supplying fresh air from outside, an end of which is enlarged in the diameter thereof so as to form an air intake 202, in which is provided a filter 203. To the intake air supply conduit 201 is connected an exhaust gas supply conduit 204, and a gas mixture circulation conduit 205 is formed downstream from the connecting portion of the intake air supply conduit 201 and the exhaust gas supply conduit 204, and through the gas mixture circulation conduit 205, exhaust gas diluted with fresh air is extracted to the downstream side by a blower 206 provided in the downstream side thereof.
Here, the reason for mixing the exhaust gas with fresh air from outside when analyzing the contents of the above-mentioned exhaust gas lies in that there is a necessity to lower a ratio of moisture within the exhaust gas by mixing with fresh air, since otherwise dew drops will be condensed due to a decrease in the temperature on the way, when trying to send a pure sample of exhaust gas to an analyzer.
Interrupting the gas mixture circulation conduit 205 in the down-stream side from the connecting portion of the above-mentioned intake air supply conduit 201 and exhaust gas supply conduit 204, there is provided a mixing device 207, and further provided is a venturi device 208 down-stream of the mixing device 207, for the purpose of maintaining the exhaust gas at a constant flow rate, as a means of conforming to the different displacements and exhaust flow rates of various kinds of engines.
Further, from the gas mixture circulation conduit 205 is divided a sampling conduit 209, in the downstream side from the above-mentioned mixing device 207 but up-stream from the above-mentioned venturi device 208, and this sampling conduit 209 is further divided into three (3) conduits, to each of which is connected one of sampling bags 211 . . . through one of joints 210 . . . , respectively.
Further, from the air supply conduit 201 leads a reference air supply conduit 220, from the flow up-stream from the connecting portion with the exhaust gas supply conduit 204, and this reference air supply conduit 220 is also divided into three (3) conduits to be connected to the sampling bags 211 . . . through one of the joints 210 . . . , respectively.
Furthermore, a lead back conduit 230 being connected to the analyzer is connected to or closed from each one of the divided conduits 209a and 220a by means of valves 230a . . . while an exhaust conduit 240 is also connected to or closed from each one of the divided conduits 209a and 220a by means of valves 240a, respectively. Still, valves 212 and 222 are provided in the up-stream side from the valves 230a and 240afor each one of the divided conduits 209a and 220a, and also a reversible pump 241 is provided in the flow of the exhaust conduit 240.
Moreover, an air supply conduit 242 is connected or joint to the above-mentioned gas mixture circulation conduit 205 just before (up-stream of) the blower 206 provided thereon.
The analyzing method in such an exhaust gas sampling apparatus according to the conventional art, the structure of which is mentioned in the above, will be described below, in particular with reference to the sampling conduit 209.
First, the pump is driven under the condition where the valves 212 are turned OPEN while the valves 230a and 240a are CLOSED, so as to introduce the diluted exhaust gas into each of the sampling bags 211 according to a measuring mode, and then the valves 212 and 240a are turned CLOSED while the valves 230a are OPEN, so as to send the diluted exhaust gas into the analyzer through the lead back conduit 230, thereby measuring carbon monoxide (CO), hydrocarbon (HC), nitrogen compounds (NO.sub.x) and so on in the exhaust gas, in accordance with the predetermined mode for measuring.
As such the measuring modes are already known as an LA-4 mode and a US06 mode, as per the regulations in the USA, for example. (There are also known a 10.15 mode and 11 mode, as per the domestic regulations in Japan.) Here, in accordance with the LA-4 mode, the contents in the exhaust gas are measured in each of the following time periods: (1) from starting of the engine up to a time point of a lapse of 504 sec., (2) from 505 seconds up to the time point of the lapse of 1,374 sec., and (3) from a re-starting after 10 minutes suspension of the engine up to the time point of the lapse of 504 sec. Accordingly, in the LA-4 mode, the exhaust gases during the above-mentioned intervals (1) to (3) must be collected into the sampling bags, respectively, and therefore there are prepared the three (3) pieces of the divided conduits and the three (3) bags.
Also, in accordance with the SU06 mode, in which the exhaust gas exhausted from the engine being at almost full throttle for ten (10) minus is reserved or stored in the sampling bag to be analyzed, only one of the sampling bags is used according to this measuring mode.
In the case where the measurement of the above LA-4 mode is conducted with an ordinary gasoline engine, the flow rate is changed for each of the intervals (1) to (3) of the LA-4 mode, while suppressing the mixture with air to as low a ratio as possible, for obtaining correct measurement values. Therefore, because the gas mixture flowing within the gas mixture circulation conduit must be controlled to be from 3 to 9 m.sup.3 /min in the flow rate thereof, the above-mentioned venturi, of a variable flow rate type, is utilized.
Further, in the US06 mode, the gas mixture must be controlled at 21 m.sup.3 /min in the flow rate thereof.
FIG. 9(a) shows an enlarged cross sectional view taken along the direction of the flow in the venturi 208 of the variable flow rate type, and FIG. 9(b) shows an enlarged cross sectional view taken in the direction orthogonal to the direction of flow, wherein the cross sectional area of flow passage 252 at a neck portion 251 is changed by shifting an external wall or a core of the venturi in a relative manner, so as to control the flow rate through it.
When completing the measurement, the valves 212 and 230a are turned CLOSED while the valves 240a are OPEN, and air is sent into the sampling bags 211 by driving the reversible pump 241 of the exhaust conduit 240 into the reverse direction of rotation. Then, the reversible pump 241 is rotated in the forward direction to discharge the exhaust gas including the air within the sampling bag. The exhaust gas within the sampling bag is completely removed by repeating this operation, so as to be prepared for the next measurement.
In the exhaust gas sampling apparatus as mentioned above, an exhaust pipe of an automobile is connected to the exhaust gas supply conduit 204 and at the same time the blower 206 is driven, then the exhaust gas is diluted by mixing with the intake air, to a concentration wherein the mixture has a mass 20 times that of the exhaust gas constituent therein, so as to eliminate the condensation of water droplets due to the decrease in the temperature of the gas. After being sent through the sampling conduit 209, as well as the conduit divided therefrom, to be then stored in the bag 211, a portion of the exhaust gas is supplied to the analyzer to be analyzed.
However, when the exhaust gas exhausted from the engine is diluted to 20 times the mass thereof, it is difficult to obtain a correct result of the measurements. In particular, with an automobile of low pollution (emissions) installed with a lean burn gasoline engine which burns diluted fuel therein, the contents of the exhaust gas discharged from it is inherently near to those of the fresh air, and the drawback mentioned above is considerable.
Further, as the sampling conduit is used a conduit made from Teflon resin or that on which Teflon resin is coated, according to the conventional art. With this, however, pollution or soot is easily absorbed on the sampling conduit and also an ingredient of hydrocarbon is exuded from the Teflon resin, therefore it is unsuitable for use in the measurement of the emissions of an automobile of a low pollution type as discussed above.
Moreover, using sampling conduit 209 as an example, of the conventional art, when completing the predetermined sampling operations, the valves 230a are turned OPEN while keeping the valves 212 . . . in the CLOSED condition, and the reversible pump 241 is driven to rotate in the reverse direction so as to supply the air into the bags 211 . . . through the exhaust pipe 230, and thereafter, the reversible pump 241 is driven to rotate in the forward direction to discharge the gas from the bags. This is repeated several times so that no exhaust gas remains within the bags or the conduits connecting to the analyzer.
However, though conducting such air purging in the above-mentioned manner, the absolutely correct measurement value cannot be expected, since it is impossible to substitute the air for the exhaust gas remaining within the sampling conduit 209 at the side up-stream of the valves 212 . . . .
Furthermore, in the conventional art, the contents of the exhausts gas remaining within the conduits are removed by the air, however, since the contents contained in the exhaust gas are very small in quantity and are similar in composition to the fresh air, in particular in the case of the measurement for the low pollution car, no guarantee can be made of the correct measurement with such air purging.
Also, in the exhaust gas sampling apparatus of the conventional art, the analysis is made by connecting the sampling bags 211 to the conduits 209a . . . at the side of a main body thereof, therefore, the conduits must be elongated, which is troublesome in a case where the analyzer is located at a position far from the exhaust gas sampling apparatus. Further, on the passage thereof is generated the water condensation into which the contents of the exhaust gas dissolve, therefore, it is impossible to obtain a correct result of the analysis.
Even in the conventional apparatus, it is of course possible to divide the sampling bags 211 from the portion of the joints 210, however it is difficult to carry a large number of the bags, and there is a possibility of mistaking the bag in which the reference air is contained for the bag in which the diluted exhaust gas is contained to be measured.
Furthermore, even if it is possible to carry them, since the exhaust gas within the bags is cooled down during transport (flow) thereof through the conduits, causing water condensation, and into the condensation is dissolved the contents of the exhaust gas, it is therefore difficult to obtain the measurement correctly.
Also, the conventional apparatus includes a problem relating to the variable type of Venturi. In the conventional apparatus, for the purpose of decreasing the flow rate of the gas mixture, the core is shifted to the left-hand side in FIG. 9(a), so as to make the area of flow passage narrower. On the contrary, it is shifted to the right-hand side for increasing the flow rate, so as to widen the area of flow passage. For enabling both the measurement of the LA-4 mode and of the US06 mode, although it is theoretically adequate to set up the diameter size of the Venturi, in particular at the neck portion thereof, so that the flow rate of 21 m.sup.3 /min can be obtained under the condition of full opening thereof, it is difficult, however, to throttle correctly if the diameter at the neck portion is large, and the control of the flow rate in the vicinity of 3 m.sup.3 /min comes to be difficult to achieve.
For this reason, in the case where the venturi of the variable flow rate type mentioned above is used, it is difficult to control the flow rate widely. Namely, the control of flow rate in the range from 3 to 21 m.sup.3 /min is impossible to achieve by use of only one of the venturi of the variable flow rate type.
As a means for solving the above problem, by providing a plurality of Venturis of a fixed flow rate type, it is possible to achieve the control of flow rate by exchanging the OPEN and CLOSED states therebetween, however when doing so, it is difficult to change the flow rate continuously therewith. For controlling it finely, the number of the venturis of the fixed flow amount types must be increased, and the structure thereof thus becomes complicated.
Furthermore, when providing the plurality of venturis of the fixed flow amount types, the flow rate becomes unstable when they are exchanged therebetween.